Various fuel delivery systems may be used to provide a desired amount of fuel to an engine for combustion. Some fuel delivery systems utilize port fuel injectors to deliver fuel to each of cylinder of the engine. Other fuel delivery systems utilize direct fuel injectors to deliver fuel directly to each cylinder of the engine. Still other engines have been described that utilize both port and direct fuel injectors for each cylinder to deliver different types of fuel to the engine.
One example is described in the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. Specifically, the Heywood et al. papers describe directly injecting ethanol into the cylinders to improve charge cooling effects, while relying on port injected gasoline to provide the majority of the combusted fuel over a drive cycle.
In one particular example, if the ethanol fuel is used up before the gasoline fuel, the engine may experience increased knock at higher boost levels. As a result, the boost level may be reduced while the engine is operated with port injected gasoline fuel.
The inventors herein have recognized several issues with this approach. For example, such an approach results in significantly reduced available boost, and thus engine output torque. Specifically, since the engine relies on port injected gasoline when the ethanol is used up, not only are the charge cooling effects of ethanol's increased heat of vaporization lost, but also the charge cooling effects of direct injection.
To address these and other issues, the inventors herein have provided a method for controlling an internal combustion engine to deliver a desired engine output. The method comprises: delivering a first fuel blend from a first fuel tank to the engine; delivering a second fuel blend from a second fuel tank to the engine, the proportion of said second fuel blend delivered to the engine to said first blend delivered to the engine being related to the desired engine output; transferring said first fuel blend from said first fuel tank to said second fuel tank to prevent an amount of said first fuel blend and said second fuel blend in said second fuel tank from falling below a predetermined level; and boosting air delivered to the engine, the amount of boosting being related to latent heat of vaporization or alcohol concentration of said second fuel blend delivered to the engine.
In this way, fuel (albeit possibly with a lower heat of vaporization) is available for direct injection even when the primary direct injection fuel (e.g. ethanol) is exhausted prior to the other fuels residing on-board the vehicle. In this way, it is possible to at least obtain charge cooling benefits of direct injection, and therefore the engine may be operated with a greater amount of boost than if using port injected gasoline. As such, even if the boost is somewhat reduced, the amount of reduction may be lowered.
Further, where fuel is transferred among two or more fuel tanks in the system, it is possible to compensate for a resulting change in composition of the fuel or fuel blend that may occur as a result of the fuel transfer. As one example, a fuel transfer may create a mixture of two or more fuels or fuel blends that exhibits a lower knock suppression capability than the fuel or fuel blend that was initially available to the direct injectors. However, since fuel delivery via the direct injectors may be maintained by the transfer of fuel, substantial knock suppression may still be realized even if the transferred fuel exhibits a lower knock suppression capability than the original fuel.
In other words, the inventors herein have recognized that direct injection of fuel can provide greater charge cooling affects than port injection, even when the fuel available to the direct injectors has a reduced latent heat of vaporization. Furthermore, the level of boost provided to the engine by a boosting device such as a turbocharger or supercharger may be optionally adjusted in response to the resulting composition of the fuel mixture that is created by the fuel transfer, thereby further reducing or eliminating engine knock.